Enhanced capacity bearing

ABSTRACT

A bearing is provided particularly suitable for a turbine and turbine related components used in the aerospace industry. More particularly, the bearing is suitable for such applications as pneumatic starters for turbines. The bearing is preferably constructed from a M50 steel, which typically has a pre-treated hardness in the range of approximately 60 to 64 Rc. With the present invention, the bearing is nitrided to obtain a case hardness of approximately 74 Rc. To achieve such a case hardness, the bearing may require nitriding over several days. In one preferred embodiment of the present invention, a pneumatic starter includes a shaft, preferably constructed from M50, that has a portion of its outer surface nitrided. The nitrided surface functions as an inner race of a bearing assembly. Nitriding the portion of the shaft extends the bearing life of the bearing due to its increased hardness.

BACKGROUND OF THE INVENTION

[0001] This invention relates to an improved bearing, and more specifically, the invention relates to an improved bearing particularly suitable for turbines used in the aerospace industry.

[0002] It is desirable to increase the life of bearings to decrease the amount of service necessary for the bearing and the components associated with the bearing. One way of increasing the bearing life is to increase the hardness of the bearing to reduce bearing wear. One way to increase bearing hardness is to utilize a premium grade steel which provides increased hardness. For example, bearing life standards are based upon an SAE 52100 steel, which typically has a hardness in the range of 60 to 62 Rc. By utilizing a premium grade steel such as M50, a hardness in the range of 64 Rc may be obtained thereby increasing the bearing fatigue life. M50 materials are frequently used in the aerospace industry for such components as turbine bearings, for which extended bearing life is particularly desirable. In spite of the increased bearing life that M50 provides, further improvements are desirable to increase bearing durability.

[0003] Increased bearing life may also be achieved by utilizing alternative materials such as ceramics. However, ceramics are an exotic material which may cost significantly more than a premium grade steel such as M50. The size of the bearing may also be increased to increase the bearing life. However, by increasing the size of the bearing weight is added and it may be more difficult to package the bearing. Alternatively, coatings may be added to the bearing surfaces to reduce wear and increase the life of the bearing. However, once the coatings wear from the bearing, the bearing life may rapidly decrease. The coating must be carefully selected to provide sufficient adhesion between the coating and the base material.

[0004] Nitriding is a known process used to increase the case hardness, or surface hardness, of a component. However, nitriding has not been applied to turbine applications, and particularly nitriding has not been applied to premium grade thru hardened steel such as M50. Prior art bearing materials does not provide adequate hardness in the event that the case hardened portion wears through. Therefore, what is needed is a hardened premium grade steel particularly suitable for aerospace applications such as turbines.

SUMMARY OF THE INVENTION AND ADVANTAGES

[0005] The present invention provides a bearing particularly suitable for a turbine and turbine related components used in the aerospace industry. More particularly, the bearing is suitable for such applications as pneumatic starters for turbines. The bearing is preferably constructed from a thru hardened M50 steel, which typically has a pre-treated hardness in the range of approximately 60 to 64 Rc. With the present invention, the bearing is nitrided to obtain a case hardness of approximately 74 Rc. To achieve such a case hardness, the bearing may require nitriding over several days.

[0006] In one preferred embodiment of the present invention, a pneumatic starter includes a planetary gear assembly having a shaft, which is the life limiting component for particular applications. The shaft is preferably constructed from M50 alloy steel that has a portion of its outer surface nitrided. The nitrided surface functions as an inner race of a bearing assembly. Nitriding the portion of the shaft extends the bearing life of the bearing due to its increased hardness.

[0007] Accordingly, a bearing is provided having an increased bearing life without utilizing alternative materials or increasing the size of the bearing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Other advantages of the present invention can be understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

[0009]FIG. 1 is a cross-sectional view of a turbine having a pneumatic starter; and

[0010]FIG. 2 is a schematic view of a nitriding process for bearings of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0011] Achieving increased bearing life and improved performance of bearings is critical for many applications. For example, increasing bearing life for bearings used in aerospace applications such as turbine starters is desirable to increase durability and reliability. In the bearing industry, bearing life standards are based upon an SAE 52100 steel, which has a hardness of typically in the range of 60 to 62 Rc. Increasing the hardness of a life limiting bearing component may have the effect of extending the overall bearing life and lengthening the service interval of the components associated with the bearing.

[0012] A turbine starter 10 commonly used in the aerospace industry is shown in FIG. 1. The starter 10 includes a portion of a planetary gear assembly 12 supported within a housing by a bearing assembly 14. Bearing assemblies typically includes an inner race, an outer race, and a plurality of rolling elements arranged between the inner and outer races to reduce the friction there between. In the embodiments shown, the inner race may be defined by an exterior surface of a gear shaft 16, and the outer race may be defined by an inner diameter of a planetary gear 18. The gear shaft 16 is supported by the planetary gear 18 and a plurality of rolling elements 20, such as needle or tapered rollers, arranged between the gear shaft 16 and planetary gear 18.

[0013] Premium grade, high speed steels are commonly used in the aerospace industries due to their increased hardness and extended life as compared to standard bearing materials such as SAE 52100 steels. M50 alloys typically have a maximum hardness of approximately 64 Rc. Despite the increased hardness and improved bearing life provided by M50 alloys, it is desirable to further extend bearing life.

[0014] Depending on the particular application, a different part of the bearing assembly may be more highly stressed than another bearing component and therefore have a shorter fatigue life. For example, the inner race of the gear shaft 16 may fatigue earlier than the other bearing components. Accordingly, it is desirable to increase the hardness of the inner race.

[0015] In one preferred embodiment, the gear shaft 16 of the pneumatic starter 10 may be nitrided. The gear shaft 16 is preferably constructed from a M50 alloy. Referring to FIG. 2, the untreated gear shaft 32 would be loaded into a chamber 34 for nitriding.

[0016] Ammonia gas or another suitable nitrogen rich mixture 36 is provided to the chamber 34. The gear shaft 32 remains in the nitrogen rich atmosphere within the chamber 34 to increase the nitrogen content of the exterior portion of the gear shaft 16 and thereby increase the case hardness. The case hardness, or depth of the hardness, is dependent upon the temperature within the chamber 34 and the time the gear shaft 16 spends within the chamber 34, among other factors. By nitriding the gear shaft 16 for several days in the chamber 34 a case hardness of approximately 74 Rc may be achieved. If the case hardened portion of the bearing wears through, the underlying unhardened M50 material provides considerable wear resistance. In this manner, the bearing life may be increased by at least three-fold.

[0017] The invention has been described in an illustrative manner, and it is to be understood that the terminology that has been used is intended to be in the nature of words of description rather than of limitation. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. 

What is claimed is:
 1. A bearing component constructed from an M50 alloy with a nitrided case having a case hardness of approximately 74 Rc.
 2. The bearing component according to claim 1, wherein said thru hardened M50 alloy has a pre-treated hardness in a range of approximately 60 to 64 Rc.
 3. The bearing component according to claim 1, wherein said bearing component is an inner race.
 4. The bearing component according to claim 3, wherein said inner race is a shaft of a pneumatic starter for a turbine engine.
 5. A method of manufacturing a bearing comprising the steps of: a) providing a bearing component constructed from a an M50 alloy; b) nitriding the bearing component to increase the nitrogen content of an exterior portion of the bearing component; and c) achieving a case hardness of approximately 74 Rc.
 6. The method according to claim 5, wherein said bearing component is an inner race.
 7. The method according to claim 6, wherein said inner race is a shaft of a pneumatic starter for a turbine.
 8. A starter comprising: a housing; planetary gear assembly supported within said housing; a bearing assembly arranged within said planetary gear assembly wherein a portion of said bearing assembly is constructed from a nitrided high speed steel.
 9. The starter according to claim 8, wherein said high speed steel is an M50 alloy.
 10. The starter according to claim 8, wherein said bearing component is an inner race.
 11. The starter according to claim 9, wherein planetary gear assembly includes a shaft with a portion of said shaft providing said inner. 